Process for producing polysulfide rubber of low-water permeability



Patented Sept. 2, 1947 UNITED STATES ii TENT OFFICE PROCESS FORPRODUCING POLYSULFIDE RUBBER OF LOW-WATER PERMEABILITY Vania No Drawing.Application July 25, 1944, Serial No. 546,573

3 Claims.

This invention relates to polysulfide synthetic rubber and polys'ulfidepolymers.

As set forth in a number of patents issued to Joseph C. Patrickincluding 1,890,191, December 6, 1932, and 2,216,044, September 24,1940, to which reference is made, these polymers may be made by reactingpolyfunctional organic compounds with alkaline polysulfides Where thefunctionality is due to the presence of two or more substituents splitoff by reaction with said polysulfide, chlorine being the typicalcommercial representative of such functional substituents. The polymersmay also be made by the reaction of polymercapto bodies with an agentwhich supplies oxygen or sulfur in active form and causes polymerformation by the removal of hydrogen and its conversion into water orhydrogen sulfide.

This invention is applicable to polysulfide polymom in general includingthose identified by the statement that they are polymers of therecurring unit O:S2-to5 (where these skeleton carbon groups are carbonatoms separated by and joined to intervening structure and adjacentcarbon atoms respectively), and. copolymers of these units; Forconvenience we shall throughout the specification and claims employ theterm polyalkylene polysulfide polymer to mean those polymeric bodiesderived from a single polyfunctional recurring unit responsive to thegeneralized formula recited above or from a mixture of two or more ofthese. Thus we intend the term polyalkylene polysulfide polymer to bedescriptive of both polymeric products and copolymeric products havingdiverse intervening structure in the above generalized formula, apractice which is common in this field.

In the manufacture of these polymers the organic'reactant or reactantsand the polymerizing reagent are commonly reacted in the presence of anaqueous dispersion medium containing a dis-. persing orp'eptizingsubstance, e. g. magnesium hydroxide, the purpose of which is tomaintain the reactants and polymer formed therefrom in a dispersedcondition. At the completion of the chemical reaction the product is inthe form of a latex which is Washed to remove water soluble impurities.From the latex the polysulfide polymer or synthetic rubber may beseparated in solid form, e. g., as acoagulum or in the form of granules.

This technique, which is, set forth in a number of Patrick patentsincluding those above identified and also Re. 19,207, reissued June 12,1934, and 1,923,392, August 22, 1934, is applicable to polysulfidepolymers in general and is not restricted to polymers having aparticular organic structure. By the same token the present invention islikewise applicable to polysulfide polymers-in general. The symbolismabove set forth is intended to identify a broad class of said genus.

Instead of separating the polymer fromthe latex and using or selling theseparated solid polymer, the latex itself may be made, used and sold asa valuable commodity. It has substantial utility as a film-forming andimpregnating material. The residual film or solid polymer left afterevaporation of the water is uniquely resistant to hydrocarbon solventsand the latex has many uses revolving about the peculiar ability of thepolymer to resist the action of organic solvents particularlyhydrocarbon solvents. For

example the latexes are valuable as film-formers in the application ofcoatings or linings for tanks and the like designed to hold variouspetroleum products including motor and aviation grade gasoline, Dieselengine fuel, crude oil, etc.

This advantage is, however, coupled with a rather serious disadvantage,to wit, poor resistance to the difiusion of water into and through thefilms. This disadvantage limits the fields of usefulness of thepolysulfide polymer latexes.

It has been recognized for a long time that if means could be devisedmarkedly to reduce the water permeability of films deposited from thoselatexes without affecting the unique resistance of those films toorganic solvents, new fields and markets would be opened up. Such anachievement would provide a coating material dispersed in the cheapestknown dispersing medium, i. e., water, and yielding a film highlyresistant not only to many organic solvents but also to water andaqueous solutions. With such a goal in View, numerous unsuccessfulefiorts have been made to solve the problem thus presented.

A principal object of the present invention is to provide a satisfactorysolution of that problem.

The merits of such an accomplishment will be obvious to those skilled inthe art. One instance of that merit relates to the solution of problemsarising in the shipping and storage of gasoline and similar petroleumproducts. While the presently available polysulfide latex coatingcompositions are satisfactory for lining concrete and wood gasolinestorage tanks which are never filled with water, they cannot be employeddirectly for lining corrosion-susceptible containers, e. g. tankers,aircraft-carrier fuel storage tanks and the like because it is necessaryto ballast the empty tanks with water on a return voyage and corrosionis caused by the water which penetrates through the fihn. Furthermore,the handling of aviation gasoline aboard ship necessitates the use of awater-displacement system for both filling and emptying the tanks toavoid explosion hazards. Accordingly, the present invention possessesthe greatest utility in afiording tank linings for such applications,where steel or galvanized steel tanks are intermittently exposed to thecorrosive action of sea water used for ballasting.

The unsuccessful attempts include the application of palliatives withoutremoving or inactivating the cause. Such expedients include latexescontaining substances such as resins, pigments, etc., intended to impartresistance to water penetration but ineffective in accomplishing thatpurpose.

The present invention goes to the root of the difiiculty and strikes atthe cause thereof. It has been found that the said cause is (to put thematter one way) the price paid for obtaining the polymer in highlydispersed condition as the discontinuous phase in a system in whichwater is the continuous phase, i. e., the cause appears to reside in thedispersing or peptizing agent, which remains in the film deposited fromthe latex by volatilization of the water. This is a hydrophilicsubstance commonly a gelatinous insoluble or slightly soluble metalliccompound, e. g., a hydroxide of calcium, magnesium, strontium, aluminum,chromium, iron, etc.

In accordance with the invention, the latex is treated with a preformedwater-soluble reagent (hereinafter more specifically described) theeffect of which is to greatly reduce the permeability of the filmdeposited from the latex by evaporation of the aqueous dispersingmedium.

A satisfactory explanation or theory to account for the phenomenaobserved and obtained cannot be offered at the present time nor is anysuch theory necessary in order to practice the invention. It isimportant, however, to note certain facts which are useful in obtainingthe maximum benefit from the invention and which throw some light on thenature of the observed phenomena.

The added reagent is a water soluble ammonium compound. It forms astable water soluble complex with the gelatinous metallic compound.After adding the reagent, the latex may be washed until most of saidcomplex and compound is removed as shown by the very low ash value ofthe washed latex. This elTect cannot be obtained by washing theuntreated latex. Moreover the reagent has the noteworthy property ofaccomplishing its valuable results without precipitating, coagulating,hydrolyzing, decomposing or otherwise adversely affecting the latex orthe particles of polymer dispersed therein. The character of the film isgreatly improved in respect of its resistance to water penetration andPassage or permeability without in any way impairing the other valuableproperties including impermeability and resistance to organic solventsand the latex is improved in respect of its normal film-formingproperties.

The finding that the cause of low water permeability resides in thepresence of the gelatinous metallic compound plus the discovery of meansto render those compounds soluble and capable of removal by washingwould lead to the normal conclusion that the lowest permeability wouldbe attained by adding the reagent and then thoroughly washing. Thathowever is not the case, i. e. latexes which have been treated and notwashed have a lower permeability than those treated and washed althoughthe latter have a permeability greatly superior to Le, lower than,untreated latexes.

.The following data illustrate this unexpected and unique behavior:

. Permeability to water (a) Untreated latex B 308 (b) Treated with 2%NH4N03 129 (0) Same as (b), washed 1 time 136 (11) Same as (b), washed 5times It thus appears that the function of the complex-forming additiveammonium compound is not simply to solubilize the cation of the originalgelatinous hydroxidebut rather to effect an actual inactivation orneutralization.

We therefore arrive at the phenomenon that the addition of awater-soluble substance to the latex and the consequent conversion ofthegelatinous metallic compound into a soluble complex conferswaterimpermeability to the films obtained from th latex and moreeffectively when the added reagent is retained than when it and itsreaction products are washed out.

This offers the apparent paradox thatthe s0- lubilization of thedispersing agent and the retention of the reaction product of thedispersing agent and the solubilizing reagent in the latex, instead ofmaking the films more permeable, decrease th permeability.

Since it is usually desirable to wash the latex product, obtained in themanufacture of polysulfide polymers, to remove the water solublesubstances, e. g., sodium chloride, obtained as a byproduct, thepreferred procedure is to wash the latex in the normal manner then addthe reagent and cause it to remain in the latex. The invention may bapplied to the latex as a part of the manufacturing process or after thelatex has been manufactured and stored or shipped. It is preferred toapply the invention as a part of the manufacturing process.

As above stated, magnesium hydroxide is commonly employed as adispersing agent. In accordance with the principle of this invention,the added inactivating reagent is one which forms a stable solublecomplex with the magnesium ion at the pH of the latex usually about 8 to10. The class of said reagent disclosed and claimed in this applicationis water soluble, ionizable ammonium compounds in general. To illustratethis class without limitation to any particular ammonium compound theremay .be mentioned ammonium hydroxide and ammonium salts, e. g., formate,carbonate, acetate, propionate, butyrate, valerate, caproate, oxalate,malonate, succinate, tartrate, citrate, chloride, nitrate, sulphate,bromide, iodide, borate, and the various ammonium phosphates, andnumerous others.

We have found that as little as 1 per cent of inactivating reagent basedon the weight of the polysulfide polymer is effective; there appears tobe an advantage, however, in employing larger amounts, up to about 10per cent. This concentration is many times the known residualconcentration of magnesium hydroxide in the latex (determined by ashanalysis) and indicates that complex formation does not go as rapidlyand completely as would be expected of a simple metathesis. We suspectthat the presence of the organic polymeric material has an interferingaction, a characteristic of many colloidal systems.

In general we prefer to employ from about2 to about 5 per cent by weightof the ammonium compound based on the weight of the polysulfide polymerin the said latex. We have found that the treated latexes are ready foruse in as short a time as one hour following the addition of theammonium compound. We generally prefer to allow an over-night contacttime of about 16 hours before proceeding with application of the treatedlatex as a tank lining composition or for othercoating purposes.

The beneficial effect of the invention ismost conclusively demonstratedby comparing the specific water permeabilities of coalesced films fromtreated and untreated latex. For this purpose we employ a testingtechnique which is a modification of th familiar Wray-Van Horst freefilm diffusion cell method. The test specimen is prepared by spreadingmultiple coats of the aqueous latex on one side of 16 ply, Ionic blankwhite poster cardboard, a highly calendered stock approximately 50 milsthick. After the so built-up latex film has coalesced and thoroughlydried, circular discs are cut to fit snugly into the inside of theretaining band of a Seal-All Mason jar lid. The coated side faces theliquid content of the pint screw-top Mason jar which is employed tocontain the test liquideither Water, aviation gasoline, or otherparticular liquid to which the permeability is to be measured. Afterintroducing 152 ml. of the selected liquid the retaining band carryingthe coated cardboard disc is screwed on tightly and the test assembly isplaced in a constant temperature, controlled humidity room operating at70 F. and 65% R. H. for three days. At the end of this time the firstweighing is made, and subsequent weighings are made at daily intervalsfor days, or until a steady state loss of weight is obtained.

The permeability is calculated as follows:

Permeability= mgins. wt. loss per dayX measured film thickness (mils)area of coated disc (square inches) It will be noted that this gives apermeability rate based on a 20 mil thick film-a figure approximatingthat at which polysulfide synthetic rubber latex films are normallemployed in lining tanks.

Inasmuch as uncoated cardboard controls pass about 2500 mgms. of Waterand more than 10,000 mgms. of aviation gasoline per day, the presence ofthe cardboard support does not significantly interfere with transferrates calculated as though a free film were being used.

The following specific examples will serve to illustrate the principlesof the invention as defined in the claims. All of the percentage figuresrecited are on a weight basis.

Exampl 1 A Washed latex derived from reaction between 3 parts ofethylene dichloride, 1 part of propylene dichloride and an excess ofsodium polysulfide, formed in the presence of magnesium hydroxide in themanner fully described in Example 1 of United States Patent 2,216,044,for example, and having a pH of about 8 to 10 was divided, and a portiontreated with 2% of ammonium nitrate based on the solids content ofpolysulfide-type synthetic rubber. The latter sample was then Washed bydecantation before use. The samples were filmed on Ionic cardboard andallowed to coalesce in the usual manner. Permeabilities to water and toaviation gasoline were determined with the'following results:

Permeability to are:

Untreated sample 188. O 7.0 Treated with 2%NH NO3 64. 8 7. 6

The marked reduction in water permeability, with no accompanyin changein the gasoline permeability, is self-evident.

Example 2 The same treated and untreated latexes described in Example 1were pigmented with 20% (by weight) of calcium titanate, andpermeabilities on the coalesced films determined in, the

. usual manner. The data follow:

Permeability to- Aviation Water Gasoline Untreated pigmented sample l.243. 0 9. 5 Treated with 2% NH4NO 71. 6 8. 8

The beneficial effect of the treatment is obvious, even on the highlypigmented latexes.

Example 3 Permeability to Aviation Water Gasoline Untreated sample 202.012.1 Treated with 1.2% NH CI 4. 6 13. 6

The remarkable improvement in water permeability rate which resultedfrom the treatment unaccompanied by Washing after addition of thereagent is strikingly evident.

In the above example water soluble ionizable ammonium compounds ingeneral may be substituted for the specific ammonium compounds thereinset forth.

What is claimed is:

l. The process of treating an aqueous dispersion of a polyalkylenepolysulfide which is a polymer of the unit (IJ (is z m a to reduce thepermeability to water of films obtained therefrom, said dispersioncontaining magnesium hydroxide as dispersing agent and having a pH of 8to 10 and having been washed with water until substantially free fromwater soluble impurities, which comprises incorporating with saiddispersion after thewashing a preformed Water soluble ionizable ammoniumcompound, the proportion of said ammonium compound being 1 to 10 percentby weight of said polymer in said dispersion, whereby a stable watersoluble complex with the magnesium cation is formed, the pH remainingwithin the pH range of the dispersion usually 8 to 10.

2. The process of claim 1 in which the Weight percent of added preformedwater soluble ionizable ammonium compound lies between 2 and 5 percent.

3. The process of claim 1 in which the said REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Name Date J age Sept. 30, 1941 Number

